Traffic flow reported MAC addresses

Hello,
I am using Traffic flow version 9 (or IPFIX). For this purpose I have installed a hEX RB750r3 configured as a bridge just before our internet gateway. This works well, except that the MAC addresses are set to the MAC addresses of the Mikrotik ports - not to the src and dst mac addresses as stored in the packets. (I checked these MAC addresses on the systems on either side of the hEX.
Any suggestions?

Thanks!